1. Field of the Invention
The present invention relates to a control apparatus for an automatic transmission for reducing an output torque of an engine when a predetermined speed range is selected.
2. Description of the Prior Art
In an automatic transmission using a planetary gear system, an input shaft of the transmission connecting to a crank shaft of an engine through a torque converter is provided with a plurality of sets of planetary gears and a plurality of frictional engaging elements for binding a plurality of rotational elements forming the planetary gear system, and a combination of engaging conditions of the frictional engaging elements is changed over hydraulically to achieve a desired speed position.
There are proposed automatic transmission control apparatus, for example, in Japanese Patent Application Laying-open No. 61-113524 and Japanese Utility Model Application Laying-open No. 3-56845, in which the transmission torque capacity of a frictional engaging element engaging at a specific speed position is set to a small value, and when the specific speed position is selected, the engine output torque is forcibly reduced independent of the manipulated variable of an accelerator pedal by a driver, thereby providing a compact and lightweight automatic transmission, especially, one which has a reduced length of the transmission shaft.
In the automatic transmission control apparatus disclosed in Japanese Patent Application Laying-open No. 61-113524, at a reverse where the transmission generally does not require large output torque, the engine output torque is reduced to prevent a vehicle from traveling with excessive output torque during backward traveling. This enables the frictional engaging element engaging at the reverse to be of small transmission torque capacity, and the driving force transmission member connecting to the frictional engaging element to be one which has low strength, thereby achieving a lightweight automatic transmission.
Furthermore, in the automatic transmission control apparatus disclosed in Japanese Utility Model Application Laying-open No. 3-56845, when the engine is in high-speed rotation, high-torque operation region with the automatic transmission at the reverse, forcibly reduces the output torque of the engine independent of the manipulated variable of the accelerator pedal by the driver, thereby preventing the vehicle from starting abruptly.
In the automatic transmission control apparatus disclosed in Japanese Patent Application Laying-open No. 61-113524 and Japanese Utility Model Application Laying-open No. 3-56845, selection of the reverse is determined by a detection signal from an inhibitor switch linked to a select lever operated by the driver, and the detection of the reverse by the inhibitor switch is normally carried out. However, no contingency is made for the case where a malfunction occurs in the reverse position detection mechanism including the inhibitor switch.
Therefore, if breaking of signal cables occurs in the inhibitor switch or the like for detecting the reverse for some reason, it is impossible to determine the selection of the reverse even if the select lever is at a reverse speed select position. As a result, even when the reverse is selected, the engine output torque is not reduced and remains at a magnitude corresponding to the manipulated variable of the accelerator pedal by the driver, and there is some fear that excessive transmission torque is applied to a frictional engaging element engaging at the reverse and driving force transmission members connecting to the frictional engaging element.